The present invention relates to an apparatus and a method for controlling an electric compressor used in a vehicle air conditioner.
A typical electric compressor adjusts the speed of a motor by using an inverter, thereby controlling the displacement of refrigerant gas discharged from the compression mechanism per unit time. In the control using an inverter, it is difficult or impossible to actuate the motor below a lower limit rotational speed. Therefore, low displacement cannot be controlled by using an inverter. Thus, when the compressor displacement must be controlled in the range that is below the value corresponding to the lowest rotational speed of the motor, the motor is alternately turned on and off at a low speed that is close to the lowest speed. This procedure is referred to on-off control. The on-off control permits the compressor to operate in a low displacement range that cannot be obtained through inverter control.
However, when the compressor operates at a low speed that is close to the lowest speed of the inverter control, the energy efficiency of the motor and the inverter is low compared to a higher speed range of the motor. Such low speed operation of the motor therefore lowers the total energy efficiency of the compressor. Particularly, since an electric compressor used in a vehicle air conditioner is driven by the vehicle battery, the energy efficiency of the compressor must be increased to extend the period in which the air conditioning can be performed without charging the battery.
Accordingly, it is an objective of the present invention to provide an apparatus and a method for controlling an electric compressor that adjusts the compressor displacement in a wide range and improves the energy efficiency of the compressor.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a control apparatus for an electric compressor that includes a motor and a compression mechanism driven by the motor is provided. The compressor forms part of a refrigerant circuit of an air conditioner. The control apparatus adjusts the rotation speed of the motor for controlling the amount of refrigerant gas discharged from the compression mechanism per unit time. When the energy efficiency of the motor is predicted to be lower than a predetermined level, the control apparatus performs on-off control by alternately turning on and off the motor. During the on-off control, a required amount of refrigerant gas discharged per unit time is obtained, and the energy efficiency of the motor in the on time is higher than the predetermined level.
The present invention also provides a control method for controlling an electric compressor that includes a motor and a compression mechanism driven by the motor. The compressor forms part of a refrigerant circuit of an air conditioner. The rotation speed of the motor is adjusted for controlling the amount of refrigerant gas discharged from the compression mechanism per unit time. When the energy efficiency of the motor is predicted to be lower than a predetermined level, on-off control is performed by alternately turning on and off the motor. During the on-off control, a required amount of refrigerant gas discharged per unit time is obtained, and the energy efficiency of the motor in the on time is higher than the predetermined level.